Method for producing bimetallic and polymetallic bodies wherein the metals are joined together by metallurgical bond

ABSTRACT

A method is described for producing cylindrical structural elements such as plugs, bolts and the like made up of at least two sections of different metals joined together by metallurgical bond. The required number of blocks are arranged in pairs within a casing to form an extrusion billet, the block of each pair being made of different metals with reciprocal metallurgical affinity. The two blocks of each pair have one flat face disposed against the corresponding face of the other block. After extrusion, the structural elements are separated from the extrusion bar by machining in such a way that the generatrices of the structural element are perpendicular to the extrusion bar and to the transition surface of the two metals. The surface of the harder metal block, before assembling into the billet casing, is machined to form ridges with symmetrical profile which when penetrating the softer metal during the extrusion are bent to form a hooked profile.

nited States Patent n 1 Boccalari [451 Nov. 26, 1974 Assignee: Comitato Nazionale per IEnergi'a Nucleare-CNEN, Rome, Italy Filed: Jan. 2, 1973 Appl. No.: 320,195

Inventor:

US. Cl 29/481, 29/475, 29/482, 29/4975 Int. Cl B23k 31/02 Field of Search 29/481, 480, 475, 482, 29/4975 [56] References Cited UNITED STATES PATENTS Primary Examiner- Richard B. Lazarus Attorney, Agent, or Firm-Stevens, Davis, Miller & Mosher 9/l971 I Boccalari 29/481 X [57] ABSTRACT A method is described for producing cylindrical structural elements such as plugs, bolts and the like made up of at least two sections of different metals joined together by metallurgical bond. The required number of blocks are arranged in pairs within a casing to form an extrusion billet, the block of each pair being made of different metals with reciprocal metallurgical affinity. The two blocks of each pair have one flat face disposed against the corresponding face of the other block. After extrusion, the structural elements are separated from the extrusion bar by machining in such a way that the generatrices of the structural element are perpendicular to the extrusion bar and to the transition surface of the two metals. The surface of the harder metal block, before assembling into the billet casing, is machined to form ridges with symmetrical profile which when penetrating the softer metal during the extrusion are bent to form a hooked profile.

9 Claims, 15 Drawing Figures METHOD FOR PRODUCING BIMETALLIC AND POLYMETALLIC BODIES WHEREIN THE METALS ARE J OINED TOGETHER BY METALLURGICAL BOND The object of this invention is the manufacturing of bimetallic or polymetallic bodies wherein the metallic components are joined to one another through a metallurgical bond.

In a preceding US. Pat. No. 3,604,102 a process was disclosed for producing bimetallictubular joints by extrusion of two tubular coaxial metal masses fitted one into the other to form a billet which masses consist of metals which show reciprocal affinity from the metallurgical standpoint.

As stated in said patent metallurgical affinity is the property whereby some metals have similar crystal lattices so that in suitable physical conditions, an interdiffusion can occur of the atoms of one metal into the atom lattice of the other metal.

Two metals which in their pure state or alloyed show reciprocal metallurgical affinity are, for example, zirconium and iron, thus these two metals can be joined together with a metallurgical bond to form abimetallic or polymetallic body.

In the field of the nuclear technology these joints can be advantageously applied in as much as by the use of them an expensive. material such as zyrconium and its alloys can be confined to that area (core) within the reactor where the neutron flux is present and where a material of low neutron absorption are required such as zirconium. By means of said transition joints the zirconium structural elements located within the reactor core can be connected to the less expensive stainless steel structures located outside of the core that is in those areas of the reactor where neutron economy is not a major problem.

This is typically the case with the fuel rods comprising a zircaloy 2 tube wherein a stack of sintered U0 ceramic bodies are enclosed which tube is fitted between a pair of header plates made of stainlee steel. (304L stainless steel has a composition of 19 percent Cr, percent Ni, 0.03 percent C, the remainder iron. Zircaloy 2 has a composition 0.02 percent Hf, 0.12 Fe, 1.46 Sn, 0.05 Ni, the remainder iron).

A similar case is that of fuel rods to which other metal components are to be connected for measuring purposes. Obviously in both the above cases the joint is to be located atthe end of each rod only and preferably in the region of the plugs at the rod ends.

However, according to the process of the above mentioned US. Patent, bimetallic or polymetallic tube segments are only produced wherein the different metal sections which make up the bimetallic or polymetallic tube segment are successively arranged along the symmetry axis of the tube and coaxial thereto. From this drawback ensues that tubular components only are obtainable through the process of said patent. Therefore in a further copending patent application assigned to the same assignor of this application, anotherprocess has been disclosed'for producing bimetallic or polymetallic bodies wherein the metallic components are joined together by metallurgical bond. Specifically bimetallic or polymetallic plugs are produced by said process which are made up to two or more coaxial sections with radial symmetry about the same axis, in which plugs the bonding of each metal to the adjoining one or ones is along a cylindrical boundary surface coaxial with the symmetry axis of the plug.

For instance, in the case of a bimetallic plug, such cylindrical boundary surface is defined at on end by a plane containing the end surface of the plug and at the other end by a plane containing the annular surface produced by the difference between the diameters of the two adjoining sections of which the plug is made.

The intersection of said boundary surface and said annular surface appears as a transition line on the latter which, when the plug is fitted into the rod tube, could face the inside of the rod tube and be exposed to corrosion and become fragile. In fact some gases anyway present within the rod particularly H are liable to preferentially corrode the area of the plug along the transition line.

It is therefore an object of this invention to provide a process for manufacturing bimetallic or polymetallic bodies having radial symmetry about a longitudinal axis and particularly a solid cylindrical form of which the cross-section may becircular or not which bodies are obtained by joining together the elements of a pair or more than a pair of masses made of different metals the metals of each pair having reciprocal metallurgical affinity.

Specifically by this process bimetallic or polymetallic plugs can be produced wherein the interface of boundary surface between two adjoining metals is intersected by the side surface only of the plug, that is by that surface only which is parallel to the plug and not by the surfaces perpendicular thereto. In this way the danger is avoided that the transition line comes in contact with any reactant within the fuel rod.

As pointed out in the above mentioned copending application, if the conventional techniques were resorted to for joining together the metal components of a plug, that is those techniques which imply the formation of liquid phases, the results would not be acceptable because in this case also remarkable amounts of intermetallic compounds would originate which are fragile by their own nature and which would impair the mechanics strength of the joint. Hence the need arises for a joining process which does not imply the formation of liquid phases but which joins the matals involved e.g., zirconium and iron by means of a bond which does not spoil the mechanical characteristics of the bonded metals.

The plug produced by the process of this invention is made up of two or more cylindrical portions which, in the specific case to be described later on, will consist simply of two portions one of stainless steel and the other of zirconium, which portions are bonded together at their reciprocally facing ends.

without causing the metal involved to pass through their liquid state.

All the above methods are characterized by the fact that at the moment when the metallurgical bond is formed the metals involved are in a plastic state. Among said methods, the extrusion technique is particularly promising.

The process which is the object of this invention although being based on the extrusion technique, substantially differs from the known processes.

Structural elements can be produced thereby which are made up of several metals, however, for the sake of clarity, the following steps of the process will be referred to structural components or bodies made of two metals only.

a. Preparing two solid semicylindrical blocks having the same length, of which each is made oa a metal showing metallurgical affinity towards the other;

b. preparing a cylindrical casing made of a malleable metal and adapted for receiving said blocks with loose clearance thereinto;

c. cleaning and degassing the blocks and easing;

d. fitting the seuicylindrical blocks into the casing with the respective surfaces to be bonded facing each other; the assembly comprising said two blocks being also called a billet;

e. evacuating the casing of all the gases therein contained and sealing it vacuumtight;

f. heating the encased billet up to its plastic state temperature;

g. extruding the billet through a circular or rectangular die whereby a cylindrical bar is obtained wherein the above mentioned semicylindrical blocks after extrusion are metallurgically bonded to one another along the reciprocally facing surfaces;

h. cutting through the extruded bar so obtained into segments of which each is so dimensioned that the final bimetallic structural component or body can be obtained from each segment by machining operations only;

i. removing the coextruded casing from the rest of each segment;

j. machining the rest of each segment to obtain the desired bimetallic body.

It is to be noted that the bimetallic body with radial symmetry obtained by this process has a symmetry axis which is also the longitudinal axis of the body perpendicular to the extrusion axis, while according to the known processes the longitudinal axis of the bimetallic body is usually parallel to the extrusion axis.

It should be understood that the encased billet can comprise more than two components and more than two different metals and that the component form can be different from semicylindrical. In fact two or more components can be used with rectangular cross-section when this is dictated by economical and/or technical criteria as it will more clearly appear from the following examples.

The main object of this invention is therefore to provide an improved process for manufacturing bimetallic or polymetallic bodies with radial symmetry about a longitudinal axis specifically for manufacturing bi- I metallic or polymetallic cylindrical bodies in which bodies the different adjoining metals are joined together by a metallurgical bond along reciprocally facing surfaces which are substantially flat, said bodies being characterized by a high quality metallurgical bond of the metals involved, by a good reproducibility, by a high reliability standard and by a relatively low manufacturing cost.

Another object of this invention is to develop a method for manufacturing bymetallic or polymetallic bodies wherein the area long which the component metals are metallurgically bonded to one another can be exactly defined and as a consequence a large amount of waste is avoided.

A further object of this invention is to provide a process for producing bimetallic or polymetallic bodies wherein the joining surfaces among the different metals have a constant form and are such that the transition line, that is the intersection line of said joining surfaces and the outer surface of the final body always appear at the peripheral surface of the body that is at that surface of the body which is parallel to its symmetry or longitudinal axis.

Still another object of this invention is a method for producing bimetallic or polymetallic bodies whereby a plurality of said bodies are obtained from a single coextruded bar and whereby the joint or joints between the different metals all have the same technological characteristic, so that the mass production and quality control of the final product are facilitated.

Further objects and advantages of this invention will become apparent from the following description and accompanying drawings of a preferred embodiment thereof. For better clarity, the embodiment illustrated as a non-limitative example by the description and drawings is referred to the manufacturing of cylindrical bimetallic bodies with circular cross-section for instance bimetallic plugs for fuel rods in a nuclear reactor in which the surface along which the two metals are metallurgically bonded together lies on a plane perpendicular to the cylinder axis in the middle thereof.

In FIG. 1 the longitudinal section of a billet is shown which is made of two semicylindrical blocks or components having equal form with their flat surfaces juxtaposed which blocks are enclosed in a vacuumtight iron casing with cylindrical circular form.

FIG. 2 shows the transverse section of the cylindrical portion of the billet of FIG. 1;

FIG. 3 illustrates an intermediate stage of the extrusion step of the billet of FIG. 1 whereby a round bimetallic coextruded bar is obtained from a circular die;

FIG. 4 shows the cross-section of the coextruded round bar obtained through the process step of FIG. 3;

FIG. 5 illustrates the process step whereby a bimetallic plug is obtained from the coextruded round bar through a machining operation; FIG. 6 shows the crosssection of the same billet of FIG. 3 ready for being extruded through a rectangular die;

FIG. 7 is a cross-section of the bimetallic bar obtained through extrusion from the billet of FIG. 6;

' FIG. 8 illustrates a different arrangement of the active elements comprising the billet of FIG. 6;

FIG. 9 is a cross-section of the bar obtained from the billet of FIG. 8 after extrusion through a circular die;

FIG. 10 illustrates an alternative form of the casing for a bimetallic billet;

FIG. 11 is a cross-section of the bar obtained from the billet of FIG. 10 after extrusion through a circular die.

FIG. 12 is a variant of the billet of FIG. 10;

FIG. 13 is a cross-section of the bar obtained from he billet of FIG. 12 after extrusion through a circular die;

FIG. 14 illustrates a billet similar to FIG. 12 butwith a different shape of the ridges provided at the surface of the harder material to be bonded to the softer material surface;

FIG. is a cross-section of the bar obtained from the billetof FIG. 14 after extrusion through a circular die.

With reference to FIG. 1, the first step for producing bimetallic bodies according to the method of this invention consists of preparing an encased billet comprising two semicylindrical components 1 and 2 of which the flat side surfaces are disposed to face each other, the two components being reciprocally compatible from the point of view of the extrusion and having reciprocal metallurgical affinity. As an example the component 1 is made of stainless steel and the component 2 is made of zirconium or an alloy thereof. Components 1 and 2 which comprise theactive portion of the billet are then assembled in a casing 3 of cylindrical form which is called also a jacket. This is a vacuumtight vessel made of a malleable metal, for instance iron, which is evacuated of any gas through connector 4 which is then sealed.

Prior to fitting components 1 and 2 into casing or jacket 3, both the components and the casing 3 along with connector 4 are degreased and'after assembly they are all hot degassed under high vacuum. The purpose of evacuating and vacuumtight sealing the encased billet is mainly for removing any contaminants from the surfaces to be bonded and thereby facilitating the metallurgical bond.

With reference to FIG. 2, which shows a crosssection of the billet of FIG. 1, it is to be noted that one of the surfaces to be bonded is provided with longitudinal indentations 5 with sharp profile which is an additional means for impruving the metallurgical bond as already first stage of the extrusion step cutinto the softer metal 2 (zirconium or an alloy thereof) and penetrate it giving rise instantly to a fresh area of contact between the two metals free from any superficial oxidation which otherwise would be a major reason for defective metallurgical bonds.

The encased sealed billet is then heated for a certain number of hours and maintained with a' range of temperature such that it attains its plastic state and then exject of this invention. The coextruded bar 6 obtained from an encased billet is then cut along planes perpenalloy thereof 2.

truded through a circular die as shown in FIG. 3 i

. whereby a composite round bar 6 is obtained as shown in FIG. 4, which indicates also the penetrating effect of ridges 5.

For carrying out the extrusion step a conventional press for metal extrusion is employed of which in FIG. 3 the mandrel 7 the container die 8, the contained die support 9 and the ram 7 only are shown. The arrow in FIG. 3 indicates the travel direction of ram 7.

Dring the very first stage of the extrusion step, the voids between the toothed surface of component and the confronting surface of component 2 are filled up due to the extruding force applied by ram 7.

In FIG. 5 a successive step is illustrated of the process for producing the bimetallic bodies which are the ob- As can be seen in FIG. 3, the transition line appears as a serrated line 19 on the side surface of the plug. Obviously for particular purposes axis b can have an orientation other than perpendicular to axis a.

For better exploiting the extruded masses comprising the starting billet 1, this, as shown in FIG. 6 can be extruded through a rectangular die or anyway through a die other than circular.

It readily appears that in this case, when a cylindrical plug is to be obtained from a segment of bar 12 (FIG. 7) there will be a more lesser waste of material than in the case of the bar of FIG. 1.

However it is known that some drawbacks arise from the use of a die other than circular which drawbacks cannot be easily overcome. They are as follows:

a twisting effect due to defective machining and finishing of the die which is the case of an helical deformation of the bar about its longitudinal axis.

because heat is stored for a longer time within the inner layers, these are maintained for a longer time to their plastic state and therefore there is a tendency of the inner layers to advance further with respect to the outer layers, which results in a deformated bar.

a rectangular die or anyway a die other than circular is more expensive than the latter.

In general, because at the extrusion temperature the two metals of the billet (although extrusioncompatible) do not show exactly the same plasticity, and therefore one of them has a tendency to flow faster than the other, the longitudinal axis of the bar is caused to deviate with respect to the nominal extrusion axis and this obviously occurs in a larger degree with a rectangular die than with circular die.

I The last of the above listed drawbacks can be substantially eliminated by resorting to a crossed arrangement of the components 1,2 comprising the active part of the billet which arrangement at least when stainless steel and zirconium or an alloy thereof are used totally compensates for the different plasticities of th two metals at the extrusion temperature. I

With reference to FIG. 8 an encased billet is prepared which is made up of four components all having substantially the same cross-section, which is a quarter of a circle, and the same length. The four components are arranged in pairs into a cylindrical casing which is divided into two semicylindrical compartments by a diametrical partition 21. It is to be observed that partition 21 may be not integral with the casing.

In each compartment a pair of said components is fitted that is a component of one metal and one of the other metal. As shown in FIG. 8 the components of the harder metal 1 are provided with longitudinal ridges on their surfaces confronting the adjacent surfaces of the softer metal 2.

From the billet of FIG. 8, after extrusion through a rectangular die, a bar 13 is obtained (see FIG. 9) on which the same operations will be carried out as on the bar of FIG. 5.

The coextruded casing will be removed either by machining or by selective chemical attack.

With the arrangement of FIGS. 8 and 9 the last of the above listed drawbacks are eliminated in as much as the metal components of each pair are interchanged with respect to the components of the other pair whereby the tendency of one half of the billet to cause the extruded bar to bend in one direction is compensated for by the tendency of the other half of the billet to cause the extruded bar to bend in the other direction.

With the aim of eliminating the first three items of the above list of drawbacks according to an additional variant of the process of this invention, a billet is prepared which comprises two components 1,2 having substantially the same cross-section which is a rectangle and the same length and a casing 14 of a cylindrical form having a prismatic cavity coaxial therewith for receiving the said components matched together thereinto as with the previous embodiments.

By this solution the extrusion die being circular, the twisting effects are eliminated which depend either on mechanical irregularities of the die or on a temperature distribution which is not uniform.

In this case also as in the preceding and following cases, the casing material (e.g., iron) can be removed from the round extruded bar 15 of FIG. 11 either by machining or by chemical action. Thereafter the bimetallic plug which is the final product of the process can be obtained through the operations already described with reference to FIG. 7.

In the case of FIG. that is when the billet comprises two components only, the effect will still be present which is due to one metal (zirconium and alloys thereof) flowing faster through the die than the other metal (stainless steel) this resulting in a bending of the bar with respect to the nominal axis of the extrusion. Such drawback can be obviated by combining the artifice of crossing the elements of one metal with the elements of the other metal to form a billet (see FIG. 8) with the use of the cylindrical casing with prismatic hole of FIG. 10.

According to a further variant of the method of this invention, the billet to be extruded for producing bimetallic bodies will be as illustrated in FIG. 12 and the extruded round bar 16 obtained therefrom will be as shown in FIG. 13.

As shown in FIG. 12, the billet of this embodiment comprises four active components all having substantially the same cross-section which is rectangular and the same length. The four components are arranged in pairs into a cylindrical casing 22 which is provided with two prismatic compartments which are separated by a diametrical partition 23 which may be integral or not with the casing. In each compartment a pair of said components is fitted which comprises a component of one metal and one of the other, the component of the harder metal 1 being provided with longitudinal ridges on that surface thereof adjacent to the softer metal 2.

From the billet of FIG. 12, after extrusion through a circular die, a round bar 16 is obtained see FIG. 13 on which the same operations will be carried out which have been mentioned with reference to FIGS. 5, 7, 9 and 11.

According to this improvement, the longitudinal ridges 17 are not symmetrical with respect to a plane containing the summit line of each ridge. In other words when sectioned by plane perpendicular to the extrusion axis, said ridges are similar to saw teeth. Under the strain of the extrusion forces, such ridges are progressively deformed to assume in cross-section the shape of a hook. Thus, in addition to the above mentioned effect of the ridges in improving the quality of the metallurgical bond, the hooked form assumed by the ridges at the end of the extrusion. adds mechanically to the strength of the bond.

This effect is further enhances if the saw teeth 17 are symmetrically disposed with respect to the longitudinal symmetry plane C see FIGS. 14 and 15 of each pair of components in the billet which is the plane where the longitudinal axis of the final body will lie.

In FIG. 15 an extruded bar 18 is shown in which the teeth 17, after the extrusion operation, have assumed a hooked shape.

A further illustration of the quantitative aspects of the process according to this invention is provided by the following non-limitative examples thereof. Two ex amples are reported in detail while the others are sum marized in Table 1.

Example I is related to the production of plugs starting from the billet of FIGS. 1 and 2.

Example II is related to the production of plugs starting from the billet of FIG. 14.

In Table I some process parameters are reported which have been adopted in a set of tests conducted for the purpose of verifying the reproducibility of the plugs obtained by this method along with the results of said tests.

EXAMPLE I This example relates to a cylindrical billet having a length of 270 mm circular cross-section of 81 mm 0 D and jacket 4 mm thick. The two semicylindrical components having the same shape and length have been fitted into the jacket with their flat surfaces confronting each other. One was made of stainless steel AISl 304L and was provided on its flat face woth longitudinal ridges of which the form is indicated by reference number 5 in FIG. 2. The ridges height was 2.5 mm. The other component was made of Zircaloy 2. Both the ac tive components of the billet have been enclosed in a soft iron casing vacuumtight sealed and evacuated to a pressure lower than 1 10' tor.

Prior to introducing it into the casing the stainless steel semicylinder and the casing have been degreased and degassed; the Zircaloy semicylinder has been degreased only but not degassed for avoiding the known phenomena of superficial oxidation at high temperature.

After heating the billet in a furnace for 2 hours at 900C it was subjected to extrusion by means of the equipment of FIG. 3 with a ram 7 velocity of 0.40 in per minute.

The following are other characteristics of the extruslon:

The ration between the corresponding transverse areas of the billet before and after extrusion was 2.8 to l. The extruded round bar had the following dimensions: 49 mm OD; 420 mm long.

The time period from the moment when the billet was withdrawn from the furnace to the moment when ram 7 started to move: 9 seconds.

The operating pressure of the press which was maintained constant during the-extrusion step: 80 Kglcm The ends of the extruded bar have been cut at right angles with respect to the bar axis; from the end sections and from a middle section of the bar three disks .have been taken 12 mm thick. After polishing they clearlyshowed the joint area between the two components which resulted reciprocally bonded by metallurgical bond.

Destructive tests and bending tests have been carried out conforming to standardized methods on bimetallic plugs obtained through machining operations according to the method of FIG. 5, the results of which test have been satisfactory.

EXAMPLE II This example relates to a cylindrical billet with circular cross-section of .81 mm OD and 270 mm long. The billet comprised four components having the same configuration as in FIG. 14, the cross-section of each component being 27.5 24.5 mm. The partition thickness was 3 mm and the ridges height'was 2.5 mm the ridges profile being in the fomi of saw teeth which were provided on the stainless steel component. The billet has been extruded according to the same method of Example I with the same process parameters.

In this way, after a machining operation as indicated in FIG 13 several cylindrical bimetallic plugs have been obtained each having a transition joint in the middle. Some of them have been subjected to a passivating treatment in autoclave with superheated steam at '400,C and at a pressure of 105 atm for 3 days.

v Tensile tests with passivated and not passivated plugs demonstrated the high quality of the metallurgical bond achieved by this method.

In Table 1 additional examples of plugs are summarized which have been obtained through extrusion according to this method and which have been subjected to the same tests ofthe Examples I and-II.

In all the recited examples the coextruded bars have been cooled in air, the extrusion speed has been the same as in Example I and the tensile tests have been carried out at ambient temperature.

Several set of tests have been carried out for the purpose of verifying the reproducibility of the process and the results obtained thereby confirmed that this method of making bimetallic plugs is highly reliable.

When polymetallic plugs are to be produced the process steps will be the same as previously described with reference to the bimetallic plugs with the exception of the step for preparing the billet.

A billet for producing polymetallic plugs is similar to the billets shown in FIGS. 12 and 14 but in each of the two compartments of the billet, instead of a pair of components, as many components are fitted as are the desired components of the final plug.

In each of the two compartments of the billet the components will be arranged in an order which is the reverse with respect to the other compartment. Obviously, in this case again, the harder of the two metals which confront each other will be provided with longitudinal ridges.

Some embodiments of the invention have been described as non-limitative examples thereof. Obviously many modifications and variants can be introduced in. said embodiments by those skilled in the art. For instance, when a plug is to be obtained as final product consisting of three sections butt-joined with metallurgical bond of which sections the first is made of the metal A, the second of the metal B and the third of a matal A, it clearly appears that a billet is to be formed wherein a block of the metal A engages with a flat side thereof a corresponding flat side of a second block made of the metal B which second block engages with another flat side thereof the flat side of a third block made of metal A. In a similar manner a plug can be obtained made up of three sections respectively consisting of three metals A, B, C arranged in this order or in the order B, A, C provided that between A and B and between B and C in the first case there exists metallurgical affinity and that such affinity exists between B and A and between A and C in the second case. Furthermore the body obtained from the composite extruded bar may have a form other than the cylindrical form il- TABLE Reference number of extrusion TENSILE STRESS TESTS ON DIFFERENT TYPER OF JOINTS SSIZ VZ steel designation diameter ultimete of the tensile joint stress (mm) (Kg/mm) reduction ratio Billet temp. "C

() Failure of the sample occurred in the Zirettloy: the joint urea remained unaltered.

What is claimed is:

l. A process for producing metal bodies particularly plugs and similar structural elements made up of at least two sections of different metals joined together by a metallurgical bond which process comprises the following steps:

a. preparing a plurality of cylindrical blocks of same length, a pair at least of which are made of metals different from one another but with reciprocal metallurgical affinity, the blocks of said pair being provided each with a flat side face equal in area to a flat side face of the other block of said pair;

b. cleaning and degassing said blocks;

c. preparing a cylindrical casing a malleable metal adatped for receiving said blocks thereinto side by side in parallel relation;

d. cleaning and degassing said casing;

e. fitting said blocks into said cylindrical casing in such a way that said faces equal in area of each pair of blocks engage each other and in such a way that no appreciable voids be left among the blocks in the casing whereby an extrusion encased billet is formed;

f. evacuating said casing of any gases and sealing it vacuum tight;

g. heating the encased billet to a temperature at which all the billet components are in their plastic state:

h. extruding the billet through a die and obtaining a cylindrical composite bar in which the metal blocks comprising the starting billet are bonded together along their reciprocally engaging surfaces;

. cutting through the extruded bar perpendicularly to its longitudinal axis to obtain bar segments long enough to contain the outline of the final product plug and removing the extruded casing or jacket by machining or by mechanical action;

. machining each segment obtained as above to produce a plug which is defined by a cylindrical side surface of which the generatrices are not parallel to the extruded bar which plug contains at least one transition or bonding surface between two different metals within the extruded bar, the line of intersection of said transition surface and the plug surface being all contained on the side surface of the plug.

2. A process according to claim 1 wherein the components of the billet are two, the machining of the bar segments provided by step (j) is carried out in such a way that the generatrices of the desired plug are perpendicular to the longitudinal axis of the bar segment and also perpendicular to the transition surface between the two metals.

3. A process as per claim 1 which process further comprises forming a plurality of longitudinal wedgelike projections on that surface of the harder metal block or blocks which are inteded for engaging the corresponding surfaces of the other blocks after the blocks are assembled together to form a billet.

4. A process as per claim 3 wherein said wedge-like projections have in cross-section a sawtooth profile the shorter side of each tooth being substantially perpendicular to the block face which it is intended to pene trate.

5. A process for producing metal plugs as per claim 1 wherein the cylindrical blocks which along with the casing or jacket make up the extrusion billet comprise two semicircular cylinders, the casing is a cylindrical circular tube with relatively thin wall, the extrusion die is circular. the extruded bar is cylindrical circular and from said segment a solid cylinder is obtained by machining of which the genearatrices are perpendicular to the longitudinal axis of the bar segment of the transition surface of the two metals.

6. A process for producing metal plugs as per claim 1 wherein the cylindrical blocks which along with the casing or jacket make up the extrusion billet comprise two semicircular cylinders, the casing or jacket is a cir cular tube with relatively thin wall, the extrusion die is rectangular. the extruded bar is a rectangular prism and from said segment a solid cylinder is obtained by machining of which the generatrices are perpendicular to the longitudinal axis of the bar segment and to the transition surface of the two metals.

7. A process for producing metal plugs as per claim 1, wherein the cylindrical blocks which along with the casing make up the extrusion billet comprise two pairs of cylinders each cylinder having a quarter of a circle crosssection, the cylinders of one pair being made of metals different from one another but with reciprocal metallurgical affinity and being provided each with a flat side face equal in area to a flat side surface of the other block with which it is engaged within the billet, the diametrically opposed cylinders being made of the same metal, the two pairs of cylinders being separated from one the other by a partition of the same metal as said casing which partition is removed along with the extruded casing during step (i) of claim 1.

8. A process for producingmetal plugs as per claim 1, wherein the cylindrical blocks which along with the casing make up the extrusion billet comprise two quadrangular prisms, the casing or jacket isa circular cylinder provided with a central prismatical cavity adapted for receiving said prisms thereinto with the respective faces reciprocally engaged which are to be bonded by metallurgical bond and the extrusion die is circular.

9. A process as per claim 1 wherein the blocks which along with the casing make up the extrusion billet comprise two pairs of quandrangular prisms the prisms of each pair being made of different metals and having each a face equal in area reciprocally engaged, the prisms diametrically opposed with respect to the billet axis being made of the same metal; the casing comprising a circular cylinder provided with two twinned central cavities into which the two pairs of prisms are received and which are separated from one the other by 

1. A process for producing metal bodies particularly plugs and similar structural elements made up of at least two sections of different metals joined together by a metallurgical bond which process comprises the following steps: a. preparing a plurality of cylindrical blocks of same length, a pair at least of which are made of metals different from one another but with reciprocal metallurgical affinity, the blocks of said pair being provided each with a flat side face equal in area to a flat side face of the other block of said pair; b. cleaning and degassing said blocks; c. preparing a cylindrical casing a malleable metal adapted for receiving said blocks thereinto side by side in parallel relation; d. cleaning and degassing said casing; e. fitting said blocks into said cylindrical casing in such a way that said faces equal in area of each pair of blocks engage each other and in such a way that no appreciable voids be left among the blocks in the casing whereby an extrusion encased billet is formed; f. evacuating said casing of any gases and sealing it vacuum tight; g. heating the encased billet to a temperature at which all the billet components are in their plastic state: h. extruding the billet through a die and obtaining a cylindrical composite bar in which the metal blocks comprising the starting billet are bonded together along their reciprocally engaging surfaces; i. cutting through the extruded bar perpendicularly to its longitudinal axis to obtain bar segments long enough to contain the outline of the final product plug and removing the extruded casing or jacket by machining or by mechanical action; j. machining each segment obtained as above to produce a plug which is defined by a cylindrical side surface of which the generatrices are not parallel to the extruded bar which plug contains at least one transition or bonding surface between two different metals within the extruded bar, the line of intersection of said transition surface and the plug surface being all contained on the side surface of the plug.
 2. A process according to claim 1 wherein the components of the billet are two, the machining of the bar segments provided by step (j) is carried out in such a way that the generatrices of the desired plug are perpendicular to the longitudinal axis of the bar segment and also perpendicular to the transition surface between the two metals.
 3. A process as per claim 1 which process further comprises forming a plurality of longitudinal wedgelike projections on that surface of the harder metal block or blocks which are inteded for engaging the corresponding surfaces of the other blocks after the blocks are assembled together to form a billet.
 4. A process as per claim 3 wherein said wedge-like projections have in cross-section a sawtooth profile the shorter side of each tooth being substantially perpendicular to the block face which it is intended to penetrate.
 5. A process for producing metal plugs as per claim 1 wherein the cylindrical blocks which along with the casing or jacket make up the extrusion billet comprise two semicircular cylinders, the casing is a cylindrical circular tube with relatively thin wall, the extrusion die is circular, the extruded bar is cylindrical circular and from said segment a solid cylinder is obtained by machining of which the genearatrices are perpendicular to the longitudinal axis of the bar segment of the transition surface of the two metals.
 6. A process for producing metal plugs as per claim 1 wherein the cylindrical blocks which along with the casing or jacket make up the extrusion billet comprise two semicircular cylinders, the casing or jacket is a circular tube with relatively thin wall, the extrusion die is rectangular, the extruded bar is a rectangular prism and from said segment a solid cylinder is obtained by machining of which the generatrices are perpendicular to the longitudinal axis of the bar segment and to the transition surface of the two metals.
 7. A process for producing metal plugs as per claim 1, wherein the cylindrical blocks which along with the casing make up the extrusion billet comprise two pairs of cylinders each cylinder having a quarter of a circle crosssection, the cylinders of one pair being made of metals different from one another but with reciprocal metallurgical affinity and being provided each with a flat side face equal in area to a flat side surface of the other block with which it is engaged within the billet, the diametrically opposed cylinders being made of the same metal, the two pairs of cylinders being separated from one the other by a partition of the same metal as said casing which partition is removed along with the extruded casing during step (i) of claim
 8. A process for producing metal plugs as per claim 1, wherein the cylindrical blocks which along with the casing make up the extrusion billet comprise two quadrangular prisms, the casing or jacket is a circular cylinder provided with a central prismatical cavity adapted for receiving said prisms thereinto with the respective faces reciprocally engaged which are to be bonded by metallurgical bond and the extrusion die is circular.
 9. A process as per claim 1 wherein the blocks which along with the casing make up the extrusion billet comprise two pairs of quandrangular prisms the prisms of each pair being made of different metals and having each a face equal in area reciprocally engaged, the prisms diametrically opposed with respect to the billet axis being made of the same metal; the casing comprising a circular cylinder provided with two twinned central cavities into which the two pairs of prisms are received and which are separated from one the other by a thin wall partition. 